Method for applying a coating to a flange of an electrochemical cell or repairing a coating applied to the same

ABSTRACT

The invention describes a method for applying an electrocatalytic or a protective coating to a metal substrate or repairing a damaged area of the same, consisting in a thermal treatment of a precursor of said catalytic coating by means of a hot air jet from a blower. The temperature of the substrate is locally controlled by means of surface temperature sensors or by an infrared measuring system. The metal substrate may be an exhausted electrode structure, in which case the reactivation is easily carried out at the plant site without any need of sending the structure to the producer. The method of the invention is particularly useful for reactivating anodes for oxygen evolution as it permits to avoid the risky procedure of detaching the anode from the current conductor.

This application is a 371 of PCT/EP98/04270 filed Jul. 9, 1998.

STATE OF THE ART

The use of electrodes obtained by coating a valve metal substrate (forexample titanium, zirconium, niobium, tantalum) with an electrocatalyticpaint is known for use in different application fields. These electrodesmay be useful in several electrolytic processes, for example for theevolution of chlorine from sodium chloride brine, as anodes for oxygenevolution in electrometallurgical processes or anodes for cathodicprotection.

U.S. Pat. No. 3,632,498 describes a general method for the production ofthis type of electrodes, which consists in applying to the valve metal aprecursor, that is a paint containing the electrocatalytic components inionic form, which is converted into the catalyst by means of a thermaltreatment in air (activation). The temperatures required for theconversion may be extremely high (300-800° C.). The most common methodfor the industrial production of these electrodes foresees, after theapplication of each paint layer, heating in oven at high temperature. Asthese electrodes usually have a very large size, the ovens have a greatthermal mass which involves high production costs and severe problemsdue to the need of maintaining a homogeneous temperature profilethroughout the whole volume. The electrodes usually comprise a frame foranchoring to the electrochemical cells wherein they are to be used.During heating in oven it is the whole electrode structure thatundergoes the thermal treatment with the consequent waste of the energyused to heat unnecessarily the frame of the electrode. However, the mostsevere disadvantage is represented by the distortions caused by saidtreatment to some particularly critical areas, such as welding andconnection points among different parts. Electrodes with a thin layer ofa catalyst which coats the valve metal offer the main advantage that atthe end of the active lifetime there is no need for substituting theelectrode but just providing for reactivation with a new catalyticpaint, as described in British Patent No. 1.324.924.

The application of the coating is a simple procedure carried out byspraying, which could be made even at the plant site if it were notnecessary to resort to large dimensions ovens capable of reaching thenecessary high temperatures, a burden which most users cannot bear, alsodue to the fact that a large number of elements should be treated inorder to justify the oven installation and operation costs. Thereforethe exhausted electrodes are usually returned to the producers to bereactivated, with remarkable additional costs for shipping and packingof the same.

In many cases re-inserting the electrode into the production cyclerequires further steps. This is the case, for example, with the anodesfor oxygen evolution used in some electrometallurgical processes whereit is extremely important that the whole surface operate at the samepotential and where the ohmic drops of the electrode structure should bekept at very low values. For this reason a current conducting structureis welded onto the active surface of the electrode, which conductivestructure consists of a metal having good conductive properties, forexample, copper coated with a valve metal. In order to reactivate thistype of electrodes, usually the current conductive structure must bedetached, as it cannot undergo the thermal decomposition treatment athigh temperature, due to the different expansion characteristics of thetwo metals. A high number of elements are severely damaged during thedetachment and must be substituted. Further, welding of the currentconductive structure to the electrode involves a strong risk of locallydamaging the catalyst and must be carried out with particular care byhighly qualified technicians. The application of paint onto a metalsurface is not limited to the case of electrodes. A particular case isthe application of catalytic paints to valve metals, as described inU.S. Pat. Nos. 4,082,900 and 4,154,897. These patents describe theapplication of a paints containing a first oxide of an element of theplatinum group and a second oxide having special characteristics toinhibit corrosion. This type of coating is particularly useful forprotecting localized areas, for example interstices and junctions wherecrevice corrosion could destroy the integrity of the element. As thethermal treatment is required only in these localized areas, the need tosubject the whole element to a thermal treatment in oven stronglypenalizes said application both under the economical and practicalstandpoints.

OBJECTS OF THE INVENTION

It is the main object of the present invention to overcome the prior artshortcomings by providing a method for applying an electrocatalytic orprotective coating to a metal substrate comprising applying a precursorof said electrocatalytic or protective coating material to the surfaceof said metal substrate and subjecting the surface to a local thermaltreatment by a hot air gun or blower to produce high temperature andkeep it under continuous control. The control of the temperature of themetal substrate is made locally by means of surface temperature sensorsor by means of infrared measuring systems.

The dimension of the surface heated by the air jet depends on the typeof nozzle applied to the blower and may vary from some squarecentimeters to some hundred square centimeters.

It is a particular object of the invention to provide a method forapplying an electrocatalytic coating onto a substrate, which may consistof an exhausted electrode and which may be carried out at the plant sitewithout any need for shipping the exhausted electrode stricture to theproducers. The method of the invention is particularly useful forreactivating anodes for oxygen evolution as it permits to avoid therisky operation of detaching the current conducting structure.

It is another object of the invention to provide a method not only forreactivating exhausted electrodes but also for treating new electrodesand elements which need a protective coating against corrosion, whereasflanges or gaskets are applied during assembling in the plant. It is afurther object to provide a method for repairing a damaged area of ametal substrate, previously provided with a coating.

The invention will be better illustrated by means of some examples,which are not to be intended as a limitation of the same.

EXAMPLE 1

A solution made of:

620 ml n-butanol

40 ml HCl 36%

300 ml butyl titanat

100 g RuCl₃ was applied by electrostatic brushing to a titaniumelectrode structure having an active surface of 1 m², upon hot picklingin oxalic acid, cleaning in a ultrasonic bath and drying.

After each application of the paint, the electrode surface was heated byan air jet at 500° C. from a Leister blower, “Robust” 7.5 kW type,provided with a rectangular nozzle, 30 cm long and 1 cm wide. Thetreatment lasted about one hour and the temperature of the metalsubstrate was kept under control by an infrared system for localmeasurement.

The electrode thus prepared was used as an anode for the electrolysis ofsodium chloride in a mercury cathode cell fed with 28% brine at a pH of2.5 and a temperature of 80° C. The cell was inserted in an industrialcircuit of cells equipped with commercial electrodes. The currentdensity was 10 kA/m²; the overvoltage of the electrode of the inventionshowed no significant difference with respect to the commercialelectrodes.

EXAMPLE 2

Two zirconium bars having the same size were degreased and pickled for 8hours in a 10% oxalic acid solution at 90° C. A paint having thefollowing composition was then applied to the bars:

30 ml TiCl₃ dissolved in water

3 g anhydrous FeCl₃

1 g FeCl₂

The first bar was subjected to thermal treatment in oven at atemperature of 600° C. for 2 hours. The second bar was subjected to athermal treatment according to the method of the invention with a hotair jet at 600° C. using the same blower of Example 1, for about onehour, the only exception being the use of thermocouples to measure thetemperature.

Each bar was connected to a cathodic protection system of steelstructures buried in the soil and both bars correctly performed forabove 1000 hours at a current density of 1000 A/m².

EXAMPLE 3

The titanium anodic flange of a bipolar element of a De Nora DD 350membrane electrolyzer, potentially subject to crevice corrosionphenomena, was painted in three subsequent applications with a solutionmade of:

3 g RuCl₃

1.74 g H₂IrCl₆

390 mg TiCl₃ from a 4% by weight hydrochloric acid solution

1 ml 2-propanol

After each application, only the painted portion was subjected to thethermal treatment according to the method of the invention with a hotair jet at 540° C. using the same blower of Example 1, for 25 minutes,the temperature of the metal substrate being kept under control by meansof an infrared system for local measurement.

The element comprising the flange thus treated was inserted and operatedin an experimental bipolar De Nora DD 350 electrolyzer comprising asecond element, the anodic flange of which had not been subjected to anytreatment against corrosion. After 3000 hours of operation the elementprotected by the catalytic paint did not show any corrosion phenomena.The anodic flange of the un-treated element appeared to be covered inlocalized areas by a pulverulent deposit which, from a chemicalanalysis, resulted to be essentially made of TiO₂.

EXAMPLE 4

The damaged coating of a flange of a bipolar element of a DD 350electrolyzer was repaired as described hereinafter. The bipolar elementcame from an industrial electrolyzer disassembled after three years ofoperation for the substitution of a membrane. During the detachment ofthe gaskets, the protective coating of the titanium flange of onebipolar element came off in a limited corner area. After careful washingwith demi water and drying, the damaged area was ground with corindonesand removing also a small quantity of the old coating along theperiphery. After another washing and drying, the ground area was treatedas described in Example 3. The new coating successfully overcome theadherence test carried out by applying a suitable scotch tape and thentearing it off. No appreciable amounts of coating were removed.

EXAMPLE 5

An anode for oxygen evolution, made of a titanium base activated by acatalytic coating and a current conducting structure made of coppercoated with titanium and directed to minimizing the ohmic drops andtherefore to keep the electrochemical potential of the anode uniform,was used in chromium plating processes and withdrawn at the end of thelifetime, degreased, sandblasted and pickled in a sulphuric acidsolution. The anode was then reactivated according to the followingprocedure:

four repeated applications of a mixture made of

100 mg/ml TaCl₅

150 mg/ml IrCl₃.3H₂O

in a 20% hydrochloric acid solution tip to obtaining a deposit of 1 g/m²of noble metal

drying at 150° C. and thermal decomposition at 500° C., after eachapplication of the above paint, by means of a hot air jet using the sameblower of Example 1.

The electrode was re-inserted in the chromium plating bath, made of 300g/l of CrO₃ and 4 g/l of H₂SO₄, wherein it worked continuously for 1500hours with the same electrochemical performances as before deactivation.

The invention has been described making reference to specificembodiments thereof. However, it must be understood that modificationsof the same are possible without departing from the spirit and scope ofthis invention. One with ordinary skill can make various changes andmodifications to this invention to adapt it to the various uses andconditions. As such, these changes and modifications are properly,equitably and intended to be within the full range of equivalents of thefollowing claims.

What is claimed is:
 1. In a process for applying an electrocatalytic orprotective metal or metal oxide coating to a flange of anelectrochemical cell comprising applying a precursor of saidelectrocatalytic or protective coating to the surface of said flange anddecomposing said precursor by means of a high temperature thermaltreatment of at least 500° C., the improvement comprising applying saidthermal treatment to a localized surface of the flange by a hot air jetcoming from a gun or a blower.
 2. The method of claim 1 wherein theflange is a valve metal.
 3. The method of claim 1 wherein said precursorcontains a corrosion inhibitor.
 4. The method of claim 1 wherein saidcatalytic coating comprises at least one metal or metal oxide selectedfrom the group consisting of Pt,Ir,Os,Pd,Rh,Ru and oxides thereof. 5.The method of claim 3 wherein said corrosion inhibitor comprises atleast one metal or metal oxide selected from the group consisting ofTi,Ta,Zr,Nb,Si,Al and oxides thereof.
 6. The method of claim 1 whereinthe temperature of the flange is controlled by an infrared system forlocal measurement.
 7. The method claim 1 wherein the temperature of theflange is controlled by a thermocouple for local measurement.